Spin-transfer torque memory self-reference read method

ABSTRACT

Self-reference reading a magnetic tunnel junction data cell methods are disclosed. An illustrative method includes applying a read voltage across a magnetic tunnel junction data cell and forming a read current. The magnetic tunnel junction data cell has a first resistance state. The read voltage is sufficient to switch the magnetic tunnel junction data cell resistance. The method includes detecting the read current and determining if the read current remains constant during the applying step. If the read current remains constant during the applying step, then the first resistance state of the magnetic tunnel junction data cell is the resistance state that the read voltage was sufficient to switch the magnetic tunnel junction data cell to.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. application Ser. No.12/390,006 filed Feb. 20, 2009 and which is based off of ProvisionalApplication No. 61/111,354 filed Nov. 5, 2008, the contents of which ishereby incorporated by reference in its entirety.

BACKGROUND

Fast growth of the pervasive computing and handheld/communicationindustry generates exploding demand for high capacity nonvolatilesolid-state data storage devices. Flash memory is one such device buthas several drawbacks such as slow access speed (˜ms write and ˜50-100ns read), limited endurance (˜10³-10⁴ programming cycles), and theintegration difficulty in system-on-chip (SoC). Flash memory (NAND orNOR) also faces significant scaling problems at 32 nm node and beyond.

Magneto-resistive Random Access Memory (MRAM) is another candidate fornonvolatile and universal memory. MRAM features non-volatility, fastwriting/reading speed (<10 ns), almost unlimited programming endurance(>10¹⁵ cycles) and zero standby power. The basic component of MRAM is amagnetic tunneling junction (MTJ). Data storage is realized by switchingthe resistance of MTJ between a high-resistance state and alow-resistance state. MRAM switches the MTJ resistance by using acurrent induced magnetic field to switch the magnetization of MTJ. Asthe MTJ size shrinks, the switching magnetic field amplitude increasesand the switching variation becomes more severe.

Spin polarization current can be used to induce magnetization switchingin MRAM designs. Spin-Torque Transfer RAM (STRAM), uses a(bidirectional) current through the MTJ to realize the resistanceswitching. The switching mechanism of STRAM is constrained locally andSTRAM is believed to have a better scaling property than theconventional MRAM. However, reading a STRAM cell is challenging as thecell is scaled down.

BRIEF SUMMARY

The present disclosure relates to spin-transfer torque random accessmemory self-reference read operations and apparatus for the same. Inparticular, present disclosure relates to a spin-transfer torque randomaccess memory self-reference read operation.

One illustrative method of reading a magnetic tunnel junction data cellincludes applying a read voltage across a magnetic tunnel junction datacell and forming a read current. The magnetic tunnel junction data cellhas a first resistance state. The read voltage is sufficient to switchthe magnetic tunnel junction data cell resistance. The method includesdetecting the read current and determining if the read current remainsconstant during the applying step. If the read current remains constantduring the applying step, then the first resistance state of themagnetic tunnel junction data cell is the resistance state that the readvoltage was sufficient to switch the magnetic tunnel junction data cellto.

Another illustrative method of self-reference reading a magnetic tunneljunction data cell includes applying a read current across a magnetictunnel junction data cell and forming a read voltage. The magnetictunnel junction data cell has a first resistance state. The read currentis sufficient to switch the magnetic tunnel junction data cellresistance. The method includes detecting the read voltage anddetermining if the read voltage remains constant during the applyingstep. If the read voltage remains constant during the applying step,then the first resistance state of the magnetic tunnel junction datacell is the resistance state that the read current was sufficient toswitch the magnetic tunnel junction data cell to.

Another embodiments includes a magnetic memory apparatus having amagnetic tunnel junction data cell that is switchable between a highresistance data state and a low resistance data state upon applicationof a spin polarized switching current and a switching current or voltagesource electrically connected to the magnetic tunnel junction data cell.A voltage or current differentiator is electrically coupled to themagnetic tunnel junction data cell to detect a read current or readvoltage change within a time interval of less than 50 nanoseconds when aswitching current or voltage is applied to the magnetic tunnel junctiondata cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1 is a cross-sectional schematic diagram of an illustrativespin-transfer torque MTJ memory unit in the low resistance state;

FIG. 2 is a cross-sectional schematic diagram of another spin-transfertorque MTJ memory unit in the high resistance state;

FIG. 3 is a schematic circuit diagram of a spin-transfer torque MTJmemory unit;

FIG. 4 is a schematic circuit diagram of an illustrative spin-transfertorque MTJ memory read detection apparatus;

FIG. 5 is an illustrative detailed signal timing graphs for the readdetection apparatus shown in FIG. 5;

FIG. 6 is a is a graph of a static R-V (resistance-voltage) curve of aspin-transfer torque MTJ memory unit where the resistance state switchesfrom the high resistance state to the low resistance state;

FIG. 7 is illustrative detailed signal timing graphs for read currentdetection at the high to low resistance state switching voltage when thespin-transfer torque MTJ memory unit is in the high resistance state;

FIG. 8 is illustrative detailed signal timing graphs for read currentdetection at the high to low resistance state switching voltage when thespin-transfer torque MTJ memory unit is in the low resistance state;

FIG. 9 is a is a graph of a static R-I (resistance-current) curve of aspin-transfer torque MTJ memory unit where the resistance state switchesfrom the high resistance state to the low resistance state;

FIG. 10 is illustrative detailed signal timing graphs for read currentdetection at the high to low resistance state switching voltage when thespin-transfer torque MTJ memory unit is in the high resistance state;

FIG. 11 is illustrative detailed signal timing graphs for read currentdetection at the high to low resistance state switching voltage when thespin-transfer torque MTJ memory unit is in the low resistance state;

FIG. 12A is a flow diagram of an illustrative self-reference readingmethod sensing a read current when applying a voltage sufficient toswitch the MTJ from a high resistance state to a low resistance state;

FIG. 12B is a flow diagram of an illustrative self-reference readingmethod sensing a read current when applying a voltage sufficient toswitch the MTJ from a low resistance state to a high resistance state;and

FIG. 13 is a flow diagram of an illustrative self-reference readingmethod sensing a read voltage.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.The definitions provided herein are to facilitate understanding ofcertain terms used frequently herein and are not meant to limit thescope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, and 5) and any range within that range.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

The present disclosure relates to spin-transfer torque memory apparatusand self-reference read methods. In particular, present disclosurerelates to self-reference reading methods to determine whether aspin-transfer torque memory unit has a high resistance state or lowresistance state data state. In many embodiments, a read current or readvoltage, sufficient to switch the resistance state of a magnetic tunneljunction data cell, is applied to a magnetic tunnel junction data cell.A resulting read voltage or current is detected and if a voltage orcurrent jump or drop is detected, the resistance state of the magnetictunnel junction data cell is determined to be the opposing data statethat the read current or read voltage was sufficient to switch themagnetic tunnel junction to. If the resulting read current or resultingread voltage remains constant, then the resistance state of the magnetictunnel junction data cell is determined to be the data state that theread current or read voltage was sufficient to switch the magnetictunnel junction to. If a resulting read voltage or resulting readcurrent jump or drop is detected, then a write back operation returnsthe magnetic tunnel junction data cell to its original resistive datastate. The disclosed method provides a large available detection signal,and fast reading speed. While the present disclosure is not so limited,an appreciation of various aspects of the disclosure will be gainedthrough a discussion of the examples provided below.

FIG. 1 is a cross-sectional schematic diagram of an illustrativespin-transfer torque MTJ memory unit 10 in the low resistance state andFIG. 2 is a cross-sectional schematic diagram of another spin-transfertorque MTJ memory unit 10 in the high resistance state. A magnetictunnel junction (MTJ) memory unit 10 includes a ferromagnetic free layer12 and a ferromagnetic reference (i.e., pinned) layer 14. Theferromagnetic free layer 12 and a ferromagnetic reference layer 14 areseparated by an oxide barrier layer 13 or tunnel barrier. A firstelectrode 15 is in electrical contact with the ferromagnetic free layer12 and a second electrode 16 is in electrical contact with theferromagnetic reference layer 14. The ferromagnetic layers 12, 14 may bemade of any useful ferromagnetic (FM) alloys such as, for example, Fe,Co, Ni and the insulating barrier layer 13 may be made of anelectrically insulating material such as, for example an oxide material(e.g., Al₂O₃ or MgO). Other suitable materials may also be used.

The electrodes 15, 16 electrically connect the ferromagnetic layers 12,14 to a control circuit providing read and write currents through theferromagnetic layers 12, 14. The resistance across the spin-transfertorque MTJ memory unit 10 is determined by the relative orientation ofthe magnetization vectors or magnetization orientations of theferromagnetic layers 12, 14. The magnetization direction of theferromagnetic reference layer 14 is pinned in a predetermined directionwhile the magnetization direction of the ferromagnetic free layer 12 isfree to rotate under the influence of a spin torque. Pinning of theferromagnetic reference layer 14 may be achieved through, e.g., the useof exchange bias with an antiferromagnetically ordered material such asPtMn, IrMn and others. The reference magnetic layer 14 can be a singleferromagnetic layer, or may include multiple layers, for example, a pairof ferromagnetically coupled ferromagnetic layers, an antiferromagneticpinning layer and a ferromagnetic pinned layer, a syntheticantiferromagnetic, or a synthetic antiferromagnetic with anantiferromagnetic layer.

FIG. 1 illustrates the spin-transfer torque MTJ memory unit 10 in thelow resistance state where the magnetization orientation of theferromagnetic free layer 12 is parallel and in the same direction of themagnetization orientation of the ferromagnetic reference layer 14. Thisis termed the low resistance state or “0” data state. FIG. 2 illustratesthe spin-transfer torque MTJ memory unit 10 in the high resistance statewhere the magnetization orientation of the ferromagnetic free layer 12is anti-parallel and in the opposite direction of the magnetizationorientation of the ferromagnetic reference layer 14. This is termed thehigh resistance state or “1” data state.

Switching the resistance state and hence the data state of the MTJmemory unit 10 via spin-transfer occurs when a current, passing througha magnetic layer of the MTJ memory unit 10, becomes spin polarized andimparts a spin torque on the free layer 12 of the MTJ 10. When asufficient spin torque is applied to the free layer 12, themagnetization orientation of the free layer 12 can be switched betweentwo opposite directions and accordingly the MTJ 10 can be switchedbetween the parallel state (i.e., low resistance state or “0” datastate) and anti-parallel state (i.e., high resistance state or “1” datastate) depending on the direction of the current.

The illustrative spin-transfer torque MTJ memory unit 10 may be used toconstruct a memory device that includes multiple MTJ memory units wherea data bit is stored in spin-transfer torque MTJ memory unit by changingthe relative magnetization state of the free magnetic layer 12 withrespect to the pinned magnetic layer 14. The stored data bit can be readout by measuring the resistance of the cell which changes with themagnetization direction of the free layer relative to the pinnedmagnetic layer. In order for the spin-transfer torque MTJ memory unit 10to have the characteristics of a non-volatile random access memory, thefree layer exhibits thermal stability against random fluctuations sothat the orientation of the free layer is changed only when it iscontrolled to make such a change. This thermal stability can be achievedvia the magnetic anisotropy using different methods, e.g., varying thebit size, shape, and crystalline anisotropy. Generally, the anisotropycauses a soft and hard axis to form in thin magnetic layers. The hardand soft axes are defined by the magnitude of the external energy,usually in the form of a magnetic field, needed to fully rotate(saturate) the direction of the magnetization in that direction, withthe hard axis requiring a higher saturation magnetic field.

FIG. 3 is a schematic diagram of an illustrative spin-transfer torqueMTJ memory unit MTJ. The spin-transfer torque MTJ memory unit MTJ iselectrically connected in series to a transistor such as, for example, aNMOS transistor. The opposing side of the spin-transfer torque MTJmemory unit MTJ is electrically connected to a bit line BL. Thetransistor is electrically coupled to a source line SL and a word lineWL. The MTJ can be modeled as a variable resistor in circuit schematic,as shown in FIG. 3.

FIG. 4 is a schematic circuit diagram of an illustrative spin-transfertorque MTJ memory apparatus to detect a voltage (or current) jump ordrop during the read operation described herein. The detection circuitcan be described as a differentiator. The magnetic tunnel junction datacell R_(MTJ) (as described above) is electrically connected to a currentsource I_(S) (or voltage source Vs) and a capacitor C is electricallybetween the magnetic tunnel junction data cell R_(MTJ) and a senseamplifier A. The sense amplifier A provides a voltage output V_(OUT).Any voltage change can be detected by the differentiator. Anillustrative detailed signal is shown in FIG. 5.

FIG. 5 illustrates application of a constant current source I_(S) and acorresponding resulting voltage drop V_(S). The voltage output V_(OUT)show three voltage spikes. A clock CLOCK is utilized to remove theunwanted initial and final voltage spikes (at the start and end of thesignal detection). The resulting voltage output V_(OUT1) indicates avoltage drop due to the magnetic tunnel junction data cell R_(MTJ)switching resistance states (from the high resistance state to the lowresistance state in this example). Thus, the read operation indicatesthat the magnetic tunnel junction data cell R_(MTJ) was in the highresistance state. A write back operation can then be performed to returnthe magnetic tunnel junction data cell R_(MTJ) to the original highresistance state.

FIG. 6 is a is a graph of a static R-V (resistance-voltage) curve of aspin-transfer torque MTJ memory unit where the resistance state switchesfrom the high resistance state to the low resistance state. Whenapplying a positive voltage on the second electrode 16 in FIG. 1 or 2,the MTJ 10 enters the positive applied voltage region in FIG. 6 andswitches from the high resistance state (FIG. 2) to the low resistancestate (FIG. 1). When applying a positive voltage on the first electrode15 in FIG. 1 or 2, the MTJ 10 enters the negative applied voltage regionin FIG. 6. The resistance of the MTJ switches from the low resistancestate (FIG. 1) to the high resistance state (FIG. 2).

FIG. 7 is illustrative detailed signal timing graphs for read currentdetection at the high to low resistance state switching voltage when thespin-transfer torque MTJ memory unit is in the high resistance state.FIG. 8 is illustrative detailed signal timing graphs for read currentdetection at the high to low resistance state switching voltage when thespin-transfer torque MTJ memory unit is in the low resistance state.

A read voltage Vs is applied across the magnetic tunnel junction datacell or spin-transfer torque MTJ memory unit. The read voltage Vs isequal to or greater than the critical voltage that is sufficient toswitch the data resistance state of the magnetic tunnel junction datacell (from the high to the low resistance state in this example). Theread voltage Vs is applied for a time duration of 0.1 to 50 nanoseconds,or from 0.1 to 25 nanoseconds, or from 01. to 10 nanoseconds. Thus theread operation is a high speed operation. During the voltage pulse, theresulting (or sensed) read current Is passing though the magnetic tunneljunction data cell is detected, as illustrated in FIG. 7 and FIG. 8.FIG. 7 illustrates the magnetic tunnel junction data cell in the highresistance state R1 and switching to the low resistance state R0. Asensed read current Is jump (increase) occurs during the read operation.FIG. 8 illustrates the magnetic tunnel junction data cell in the lowresistance state R0. A sensed read current Is remains constant duringthe read operation. In other embodiments, the read voltage is equal toor greater than the critical voltage that is sufficient to switch thedata resistance state of the magnetic tunnel junction data cell from thelow to the high resistance state.

FIG. 9 is a is a graph of a static R-I (resistance-current) curve of aspin-transfer torque MTJ memory unit where the resistance state switchesfrom the high resistance state to the low resistance state. FIG. 10 isillustrative detailed signal timing graphs for read voltage detection atthe high to low resistance state switching current when thespin-transfer torque MTJ memory unit is in the high resistance state.FIG. 11 is illustrative detailed signal timing graphs for read voltagedetection at the high to low resistance state switching current when thespin-transfer torque MTJ memory unit is in the low resistance state.

A read current Is is applied across the magnetic tunnel junction datacell or spin-transfer torque MTJ memory unit. The read current Is isequal to or greater than the critical current that is sufficient toswitch the data resistance state of the magnetic tunnel junction datacell (from the high to the low resistance state in this example). Theread current Is is applied for a time duration of 0.1 to 50 nanoseconds,or from 0.1 to 25 nanoseconds, or from 0.1 to 10 nanoseconds. Thus theread operation is a high speed operation. During the current pulse, theresulting (or sensed) read voltage Vs passing though the magnetic tunneljunction data cell is detected, as illustrated in FIG. 10 and FIG. 11.FIG. 10 illustrates the magnetic tunnel junction data cell in the highresistance state R1 and switching to the low resistance state R0. Asensed read voltage Vs drop (decrease) occurs during the read operation.In many embodiments the voltage change can be 100 mV or more. FIG. 11illustrates the magnetic tunnel junction data cell in the low resistancestate R0. A sensed read voltage Vs remains constant during the readoperation. In other embodiments, the read current is equal to or greaterthan the critical current that is sufficient to switch the dataresistance state of the magnetic tunnel junction data cell from the lowto the high resistance state.

FIG. 12A is a flow diagram of an illustrative self-reference readingmethod sensing a read current when applying a voltage sufficient toswitch the MTJ from a high resistance state to a low resistance state.The method includes applying a read voltage across a magnetic tunneljunction data cell and forming a read current at block M1. The magnetictunnel junction data cell having a first resistance state and the readvoltage is sufficient to switch the magnetic tunnel junction data cellresistance (from the high to the low resistance state, in this example).At block M2 the read current is detected. Then the method includesdetermining if the read current remains constant during the applyingstep at block C3. If the read current remains constant during theapplying step, then the first resistance state of the magnetic tunneljunction data cell is the resistance state that the read voltage wassufficient to switch the magnetic tunnel junction data cell to (the lowresistance state, in this example) at block D2. If the read currentchanges (increases, in this example) the first resistance state is theopposing resistance state (the high resistance state, in this example)at block D1 and the high resistance state is written back to themagnetic tunnel junction data cell at block M3.

FIG. 12B is a flow diagram of an illustrative self-reference readingmethod sensing a read current when applying a voltage sufficient toswitch the MTJ from a low resistance state to a high resistance state.The method includes applying a read voltage across a magnetic tunneljunction data cell and forming a read current at block M4. The magnetictunnel junction data cell having a first resistance state and the readvoltage is sufficient to switch the magnetic tunnel junction data cellresistance (from the low to the high resistance state, in this example).At block M5 the read current is detected. Then the method includesdetermining if the read current remains constant during the applyingstep at block C4. If the read current remains constant during theapplying step, then the first resistance state of the magnetic tunneljunction data cell is the resistance state that the read voltage wassufficient to switch the magnetic tunnel junction data cell to (the highresistance state, in this example) at block D4. If the read currentchanges (increases, in this example) the first resistance state is theopposing resistance state (the high resistance state, in this example)at block D3 and the low resistance state is written back to the magnetictunnel junction data cell at block M6.

FIG. 13 is a flow diagram of an illustrative self-reference readingmethod sensing a read voltage. The method includes applying a readcurrent across a magnetic tunnel junction data cell and forming a readvoltage at block M11. The magnetic tunnel junction data cell having afirst resistance state and the read current is sufficient to switch themagnetic tunnel junction data cell resistance (from the high to the lowresistance state, in this example). At block M12 the read voltage isdetected. Then the method includes determining if the read voltageremains constant during the applying step at block C13. If the readvoltage remains constant during the applying step, then the firstresistance state of the magnetic tunnel junction data cell is theresistance state that the read current was sufficient to switch themagnetic tunnel junction data cell to (the low resistance state, in thisexample) at block D12. Otherwise the first resistance state is theopposing resistance state (the high resistance state, in this example)at block D11 and the high resistance state is written back to themagnetic tunnel junction data cell at block M13.

In other embodiments, the read current is sufficient to switch themagnetic tunnel junction data cell resistance from the low to the highresistance state. In these embodiments, if the read voltage remainsconstant during the applying step, then the first resistance state ofthe magnetic tunnel junction data cell is the low resistance state. Ifthe read voltage does not remain constant or changes (increases in thisexample) the first resistance state is the opposing resistance state(the high resistance state, in this example) and the high resistancestate is written back to the magnetic tunnel junction data cell atblock.

Thus, embodiments of the SPIN-TRANSFER TORQUE MEMORY SELF-REFERENCE READMETHOD are disclosed. The implementations described above and otherimplementations are within the scope of the following claims. Oneskilled in the art will appreciate that the present disclosure can bepracticed with embodiments other than those disclosed. The disclosedembodiments are presented for purposes of illustration and notlimitation, and the present invention is limited only by the claims thatfollow.

1. A method of self-reference reading a magnetic tunnel junction datacell, comprising: applying a read voltage across a magnetic tunneljunction data cell and forming a read current, the magnetic tunneljunction data cell having a first resistance state, the read voltagebeing sufficient to switch the magnetic tunnel junction data cellresistance; detecting the read current; and determining the resistancestate of the magnetic tunnel junction data cell, wherein if the readcurrent remains constant during the applying step, then the firstresistance state of the magnetic tunnel junction data cell is theresistance state that the read voltage was sufficient to switch themagnetic tunnel junction data cell to, and if the read current changesduring the applying step, then the first resistance state of themagnetic tunnel junction data cell is a second resistance state.
 2. Amethod according to claim 1, wherein the applying step has a timeduration in a range from 0.1 to 50 nanoseconds.
 3. A method according toclaim 1, wherein the applying step has a time duration in a range from0.1 to 25 nanoseconds.
 4. A method according to claim 1, wherein theread voltage is sufficient to switch the magnetic tunnel junction datacell from a high resistance state to a low resistance state, and thefirst resistance state is the low resistance state and the secondresistance state is the high resistance state.
 5. A method according toclaim 1, wherein the read voltage is sufficient to switch the magnetictunnel junction data cell from a low resistance state to a highresistance state, and the first resistance state is the high resistancestate and the second resistance state is the low resistance state.
 6. Amethod according to claim 1, further comprising determining if the readcurrent increases during the applying step, and if the read currentincreases during the applying step the first resistance state of themagnetic tunnel junction data cell is a high resistance state and thesecond resistance state is the low resistance state.
 7. A methodaccording to claim 6, further comprising writing back the highresistance state to the magnetic tunnel junction data cell.
 8. A methodaccording to claim 1, further comprising determining if the read currentdecreases during the applying step, and if the read current decreasesduring the applying step the first resistance state of the magnetictunnel junction data cell is a low resistance state and the secondresistance state is a high resistance state.
 9. A method according toclaim 8, further comprising writing back the low resistance state to themagnetic tunnel junction data cell.
 10. A method according to claim 1,wherein the magnetic tunnel junction data cell is a spin-transfer torquemagnetic tunnel junction data cell.
 11. A method of self-referencereading a magnetic tunnel junction data cell, comprising: applying aread current across a magnetic tunnel junction data cell and forming aread voltage, the magnetic tunnel junction data cell having a firstresistance state, the read current being sufficient to switch themagnetic tunnel junction data cell resistance; detecting the readvoltage; and determining if the read voltage remains constant during theapplying step, wherein if the read voltage remains constant during theapplying step, then the first resistance state of the magnetic tunneljunction data cell is the resistance state that the read current wassufficient to switch the magnetic tunnel junction data cell to, and ifthe read voltage changes during the applying step, then the firstresistance state of the magnetic tunnel junction data cell is a secondresistance state.
 12. A method according to claim 11, wherein theapplying step has a time duration in a range from 0.1 to 50 nanoseconds.13. A method according to claim 11, wherein the read current issufficient to switch the magnetic tunnel junction data cell from a highresistance state to a low resistance state, and the first resistancestate is the low resistance state and the second resistance state is thehigh resistance state.
 14. A method according to claim 11, wherein theread current is sufficient to switch the magnetic tunnel junction datacell from a low resistance state to a high resistance state, and thefirst resistance state is the high resistance state and the secondresistance state is the low resistance state.
 15. A method according toclaim 11, further comprising determining if the read voltage decreasesduring the applying step, and if the read voltage decreases during theapplying step the first resistance state of the magnetic tunnel junctiondata cell is a high resistance state and the second resistance state isa low resistance state.
 16. A method according to claim 15, wherein theread voltage decreases by more than 100 mV.
 17. A method according toclaim 15, further comprising writing back the high resistance state tothe magnetic tunnel junction data cell.
 18. A method according to claim11, further comprising determining if the read voltage increases duringthe applying step, and if the read voltage increases during the applyingstep the first resistance state of the magnetic tunnel junction datacell is a low resistance state and the second resistance state is thehigh resistance state.
 19. A method according to claim 18, furthercomprising writing back the low resistance state to the magnetic tunneljunction data cell.
 20. A method according to claim 11, wherein themagnetic tunnel junction data cell is a spin-transfer torque magnetictunnel junction data cell.